This invention relates generally to extreme ultraviolet lithography mask blanks.
In extreme ultraviolet lithography, a mask is formed from a blank. The blank provides a reflective surface which defines features. Extreme ultraviolet radiation is shined on the blank and is reflected therefrom to transfer features from the blank to a semiconductor wafer in a repeatable fashion.
Generally, extreme ultraviolet lithography masks are reflective masks fabricated by depositing interference multilayers such as molybdenum and silicon in alternating layers. The very top ending layer is referred to as a capping layer. Typically a silicon layer is used as a capping layer.
The thicker silicon capping layer is needed because of the mask patterning process control requirements. In the mask patterning process, the silicon capping layer serves as the etch stop layer for the buffer layer etch. During the buffer layer etch, when the etch selectivity to the multilayer capping layer is low, the capping layer is partially and non-uniformly removed. For example, one promising buffer layer for extreme ultraviolet lithography mask patterning is silicon dioxide. However, the etch selectivity to the silicon capping layer in a square mask etcher is rather low, for example, about 3 to 1.
Thus, there is a need for better ways to make blanks for extreme ultraviolet lithography.